1. Field of the Invention
The present invention relates to electric power generation control devices for motor vehicles, and more particularly, to techniques for controlling the power generation voltage of an electric power generator for charging electricity storage means mounted on a vehicle.
2. Description of the Related Art
Generally, a motor vehicle is equipped with a battery (electricity storage means) as a source of electric power to be supplied to electrical devices such as lamps, and also with an electric power generator (alternator) driven by the engine of the vehicle to charge the battery. The electric power generator has its power generation voltage controlled so that the charged capacity of the battery may become larger than or equal to a target value (target charged capacity). In this type of vehicle, energy regeneration is often carried out. Specifically, during deceleration, kinetic energy is converted to electric energy to be stored in the battery, and the energy stored in the battery is used during the time other than the deceleration, thereby saving energy. Also, in order to determine with accuracy the charged capacity of the battery, a technique has been developed wherein the battery is once fully charged when the engine is started, and at the moment the battery charging current is saturated, the battery is judged to be fully charged.
With this full charge determination method, the battery is not judged to be fully charged until the battery charging current becomes saturated after the start of the engine, and thus the charged capacity of the battery cannot be accurately determined in the meantime, making it difficult to set a suitable charging voltage. Accordingly, the regeneration efficiency lowers, resulting in lowering of the fuel efficiency. This problem is noticeable especially in the case where the engine is started and stopped repeatedly at short intervals of time and thus the full charge determination is made frequently.
As a conceivable solution, a method may be adopted wherein the target charged capacity of the battery is set to a large value beforehand in order to expedite the full charge determination. If the target charged capacity is set high in advance, however, the situation where charging of the battery is effected while the charged capacity is relatively high occurs more frequently. Consequently, the battery charging efficiency lowers and the regeneration efficiency does not improve much as intended.
To solve the above problem, a control device has been proposed in Unexamined Japanese Patent Publication No. 2006-230102 in which, instead of making the aforementioned full charge determination at the start of the engine, the engine stoppage time is measured and the charged capacity of the battery is estimated by subtracting a discharged capacity corresponding to the stoppage time from the previously measured capacity.
In the control device disclosed in the patent publication, however, the charged capacity of the battery is estimated simply on the basis of the engine stoppage time, and accordingly, there is the possibility that the charged capacity of the battery is estimated to be higher than an actual capacity. If the battery is charged on the basis of the estimated charged capacity higher than an actual capacity and the power generation voltage remains lower than the required voltage as a result, the charged capacity gradually lowers and the battery becomes over-discharged, causing engine start failure or reduction in the service life of the battery.